Length measuring apparatus

Abstract: A length measuring apparatus that uses an ultrasonic magnetostrictive delay line can detect a position with high accuracy by utilizing a reflected wave positively. In a measuring apparatus using an ultrasonic magnetostrictive delay line in which a wave transmitter (4) is disposed at one end of an ultrasonic magnetostrictive delay line (1), and the other end of the ultrasonic magnetostrictive delay line (1) is made an open end, a detecting coil (3) is wound around the outer periphery of the ultrasonic magnetostrictive delay line (1) over the whole area of the measuring range, a magnet piece (2) is disposed so as to become slidable along the ultrasonic magnetostrictive delay line (1) an a position of the magnet piece (2) is measured on the basis of a delay time of a detection pulse ( B ) produced in the detection coil (3) relative to a drive pulse ( A ) applied to the wave transmitter (4), a period (T₁) of the drive pulse ( A ) applied to the wave transmitter (4) is matched with a time in which a reflected wave ( C ) is generated at the ultrasonic magnetostrictive delay line (1) at its one end in which the wave transmitter (4) is disposed.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a length measuring apparatus employing an ultrasonic magnetostrictive delay line for use in measuring a level surface of a gasoline in a gasoline tank, detecting a piston rod stroke when assembled into a hydraulic cylinder or detecting positions of movable parts of a variety of industrial machines and so on. Description of the Related Art Length measuring apparatus that have hitherto been proposed use an ultrasonic magnetostrictive delay line whose length is extended or reduced with the application of a magnetic field. In the conventional length measuring apparatus, a non-reflection supporting member made of a shock-absorbing material, such as a rubber or the like, is attached to respective or one end portion of the ultrasonic magnetostrictive delay line in order to damp an undesired reflected wave generated by ultrasonic vibration. In the above conventional length measuring apparatus using the ultrasonic magnetostrictive delay line, however, it is difficult to damp the undesired reflected wave completely. Thus, reflected ultrasonic waves cross each other to interfere with a detection pulse so that the detection pulse is disturbed. There is then the disadvantage that position cannot be detected with high accuracy. Further, if strong ultrasonic vibration were generated in order to obtain a stable and large detection pulse, it would become impossible to damp reflected waves and to dissipate the reflected waves gradually. Furthermore, even when the undesired reflected wave is completely removed by using the non-reflection supporting member as described above, it is unavoidable that the whole of the length measuring apparatus becomes large in size due to the provision of the non-reflecting supporting member. An alternative length measuring apparatus is disclosed in EP-A-0363517. This apparatus comprises an amorphous metal strip, a first pulse injection coil disposed at one end of the strip, a detection coil wound around the strip and a magnet located adjacent the strip and moveable relative to the strip. The other end of the strip spaced from the pulse injection coil may be either open or provided with a second pulse ejection coil. With the other end provided with a second injection coil, both the first and second injection coils generate a pulse, both pulses being used to provide an indication of the distance of the magnet from the end of the strip. With the other end open, a pulse generated by the first pulse ejection coil is reflected from the open end, the generated pulse being used to provide an indication of the distance of the magnet from one end of the strip and the reflected pulse being used to provide an indication of the distance of the magnet from the other end of the strip. In DE-A-3131455 there is disclosed a magnetostrictive path measuring device which comprises an electrical conductor, a tube of magnetostrictive material surrounding the conductor, an ultrasonic transducer, and two permanent magnets, one of which is fixed relative to the tube and the other of which is movable relative to the tube. In operation a pulse is supplied to the electrical conductor which travels along the conductor relative to the tube. When the pulse passes each magnet an ultrasonic pulse is generated, the time taken for each ultrasonic pulse to reach the transducer being indicative of the position of a particular magnet. The stationary magnet is used as a reference so as to enable the position to be calculated independently of the velocity of the pulse. In JP-A-5722512 there is disclosed a position detecting device which comprises an ultrasonic transmissive member, a coil wound around the transmissive member and a permanent magnet which is movable relative to the transmissive member. An ultrasonic pulse from a cable transmitter located at one end of the ultrasonic member propagates through an internal part of the member, and when it reaches a part corresponding to the position of the magnet a voltage is generated in the coil owing to the cooperation between the magnet and the coil. In an operational circuit, with the input signal from the transmitter as reference, the time needed for the ultrasonic wave to reach a wave receiver located at an opposite end of the ultrasonic member and the time needed for it to reach the magnet are calculated, and a signal corresponding to the position of the magnet is output. In US-A-4510587 there is disclosed an ultrasonic displacement path measuring system which comprises a magnetostrictive conductor, a transmitter, a receiver and a magnet movable relative to the conductor. A pulse injected into the conductor cooperates with the magnet so as to generate an echo pulse indicative of the position of the magnet. Displacement path and time oscillator pulses are counted by two counter groups, and from the state of these counters the velocity and the velocity-corrected position of the magnet are calculated. It is an object of the present invention to provide an improved length measuring apparatus in which the aforesaid shortcomings and disadvantages encountered with the prior art can be eliminated. More specifically, it is an object of the present invention to provide a length measuring apparatus using an ultrasonic magnetostrictive delay line in which a position of an object to be measured can be detected with high accuracy by positively utilizing a reflected wave without utilizing a non-reflection supporting member. It is another object of the present invention to provide a length measuring apparatus using an ultrasonic magnetostrictive delay line which can be miniaturized. According to the present invention, there is provided a length measuring apparatus as specified in claim 1 hereinafter. A better understanding of other objects, features, and advantages of the present invention can be gained from a consideration of the following detailed description of illustrative embodiments, in conjunction with the figures of the accompanying drawings, in which: FIG. 1 is a schematic block diagram showing a first embodiment of a length measuring apparatus using an ultrasonic magnetostrictive delay line according to the present invention; FIG. 2 is a schematic block diagram showing a second embodiment of the length measuring apparatus using an ultrasonic magnetostrictive delay line according to the present invention; and FIG. 3 is a diagram of waveforms of a drive pulse, a detection pulse and a reflected wave used in the present invention, and to which reference will be made in explaining operation of the length measuring apparatus according to the present invention, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A length measuring apparatus using an ultrasonic magnetostrictive delay line according to the present invention will hereinafter be described in detail with reference to FIGS. 1 through 3. FIG. 1 shows a first embodiment of the measuring apparatus according to the present invention. As shown in FIG. 1, there is provided an ultrasonic magnetostrictive delay line 1. A magnet piece member 2 whose distance D from one end of the ultrasonic magnetostrictive delay line 1 is to be measured is opposed to the ultrasonic magnetostrictive delay line 1 with a predetermined distance therefrom so as to become slidable along the length direction of the ultrasonic magnetostrictive delay line 1. A detection coil 3 is wound about the ultrasonic magnetostrictive delay line 1 over the whole area of the length measuring range. A wave transmitter 4 is disposed at one end of the ultrasonic magnetostrictive delay line 1. A drive pulse generator 5 is adapted to apply a predetermined drive pulse to the wave transmitter 4. A detector 6 detects an induced voltage generated in the detection coil 3. A calculator 7 receives the drive pulse from the drive pulse generator 5 and the detection pulse from the detector 6 and then calculates a delay time of the detection pulse relative to the drive pulse to thereby output a signal corresponding to the delay time thereof, i.e., signal corresponding to a moving distance of the magnet member 2 is output from an output terminal e . In the apparatus shown in FIG. 1, when an ultrasonic vibration is applied to the ultrasonic magnetostrictive delay line 1 from the wave transmitter 4 by the drive pulse A applied thereto from the drive pulse generator 5, the ultrasonic wave is propagated in the ultrasonic magnetostrictive delay line 1. At that time, a pulse voltage is not induced in the detection coil 3 by only the detection coil 3. If the magnet piece member 2 is disposed close to the ultrasonic magnetostrictive delay line 1, then the ultrasonic magnetostrictive delay line 1 is partially magnetized by the magnet piece member 2. Therefore, due to a so-called reverse magnetostriction phenomenon (or VILLARI effect) in which a magnetic permeability of the ultrasonic magnetostrictive delay line 1 is reduced when the ultrasonic vibration passes the magnetized portion of the ultrasonic magnetostrictive delay line 1, a detection pulse B based on the induced voltage is generated in the detection coil 3. The detection pulse B is generated twice by the reflection on the open end of the ultrasonic magnetostrictive delay line 1 as shown in FIG. 3. The first detection pulse B is detected by the detector 6. When the magnet piece 2 is moved along the ultrasonic magnetostrictive delay line 1, a delay time of the detection pulse B relative to the drive pulse A is changed in proportion to a moving distance of the magnet piece 2 from one end of the ultrasonic magnetostrictive delay line 1. Accordingly, the drive pulse A from the drive pulse generator 5 and the first detection pulse B from the detector 6 are supplied to the calculator 7. Then, the calculator 7 calculates the delay time of the detection pulse B relative to the drive pulse A and outputs from the output terminal e a delay time, i.e., an output signal corresponding to a moving distance of the magnet piece 2. The ultrasonic vibration transmitted from the wave transmitter 4 is propagated in the inside of the ultrasonic magnetostrictive delay line 1, repeatedly reflected from one to the other end thereof and then progressively damped to disappear. Since a length L of the ultrasonic magnetostrictive delay line 1 is known beforehand, the apparatus is arranged such that a period T 1 in which the drive pulse A is generated by the drive pulse generator 5 is set to be the same as the period in which the reflected wave C based on the ultrasonic vibration travels through the length L of the ultrasonic magnetostrictive delay line 1. When the first reflected wave C reflected on the open end of the ultrasonic magnetostrictive delay line 1 travels the length L, arrives at and reflected on the ultrasonic magnetostrictive delay line 1 at its end portion where the wave transmitter 4 is provided, the condition of the drive pulse generator 5 is set to generate the next drive pulse A such that the wave transmitter 4 is driven to generate a new ultrasonic wave whose phase becomes the same as that of the first reflected wave C . As described above, a strong ultrasonic vibration with phases matched can be obtained by overlapping the reflected wave C on a new ultrasonic wave by the new or the second driving pulse A . Accordingly, a detected wave of large vibration with satisfactory S/N (signal-to-noise ratio) and therefore the reflected wave need not be suppressed by the non-reflection supporting member, whereby the measuring apparatus can be miniaturized by a space of the non-reflection supporting member. A second embodiment of the length measuring apparatus using the ultrasonic magnetostrictive delay line 1 according to the present invention will be described below in detail with reference to FIG. 2. In FIG. 2, like parts corresponding to those of FIG. 1 are marked with the same references and therefore need not be described in detail. As shown in FIG. 2, there is provided a reflected wave detector 8. The length measuring apparatus according to the second embodiment shown in FIG. 2 is different from the measuring apparatus using the ultrasonic magnetostrictive delay line shown in FIG. 1 in that the reflected wave detector 8 detects that the first reflected wave C shown in FIG. 3 is returned to the wave transmitter 4, the detected output from the reflected wave detector 8 is supplied to a drive pulse generator 9 and that the drive pulse A synchronized with the reflected wave C is generated by the drive pulse generator 9 to thereby drive the wave transmitter 4. The drive pulse A from the drive pulse generator 9 is supplied to the calculator 7, too. The reflected wave C is detected and the drive pulse A synchronized with the reflected wave C is generated so that, when the length L of the ultrasonic magnetostrictive delay line 1 is varied, the drive period of the drive pulse A need not be adjusted in accordance with the length L of the ultrasonic magnetostrictive delay line 1. Also, when the drive period of the drive pulse A is changed with the change of temperature, the phase of the reflected wave C and that of the new ultrasonic wave can be prevented from being displaced. According to this embodiment, since the wave transmitter 4 has a function to detect the reflected wave C , it is possible to measure the time T 1 in which the ultrasonic vibration travels back and forth within the ultrasonic magnetostrictive delay line 1. In this case, T 2 assumes a time from the drive pulse A to the detection pulse B, L assumes the length of the ultrasonic magnetostrictive delay line 1, D assumes a distance from the wave transmitter 4 to the magnet piece 2, and V assumes a velocity at which the ultrasonic vibration propagates within the ultrasonic magnetostrictive delay line 1. Then, T 1 = 2L/V T 2 = D/V Eliminating V from the above equations (1) and (2) yields the following equations (3) and (4): T 2 /T 1 = D/2L D= T 2 · 2L/T 1 More specifically, even when the velocity V at which the ultrasonic wave vibration propagates is changed with the change of temperature, the position D of the magnet piece 2 can be calculated by calculating the above equations (3) and (4) regardless of the velocity V. According to the embodiments of the present invention, since the phase of the reflected wave C and the phase of the drive pulse A are matched by making the period T 1 of the drive pulse A applied to the wave transmitter 4 coincident with the time in which the ultrasonic wave generated by the drive pulse A applied to the ultrasonic magnetostrictive delay line 1 from the wave transmitter 4 is reflected at the open end of the ultrasonic magnetostrictive delay line 1 and returned to the wave transmitter 4, the ultrasonic wave generated by the drive pulse A and the reflected wave C are synthesized at the same timing point. Therefore, an undesired reflected wave is not generated and the detection pulse B with high accuracy can be detected. Further, according to the embodiments of the present invention, since the reflected wave C is detected by the wave transmitter 4 and the reflected wave detector 8 and the phase of the reflected wave C and that of the drive pulse A are matched with each other by applying the drive pulse A synchronized with the reflected wave C to the wave transmitter 4, the detection pulse B with high accuracy can be detected constantly. Furthermore, according to the embodiments of the present invention, since a ratio between the time T 1 from the drive pulse A to the next drive pulse A and the time T 2 from the drive pulse A to the detection pulse B is calculated and the position signal corresponding to the distance D from the wave transmitter 4 to the magnet piece 2 is output, the position of the magnet piece 2 can be detected independently of the velocity V at which the drive pulse A is propagated. Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications could be effected therein by one skilled in the art without departing from the scope of the novel concepts of the invention as defined in the appended claims.

Claims: A length measuring apparatus comprising: an ultrasonic magnetostrictive delay line (1); a wave transmitter (4) provided at one end of said ultrasonic magnetostrictive delay line (1), the other end of said ultrasonic wave magnetostrictive delay line (1) being open; a detection coil (3) wound around the outer periphery of said ultrasonic magnetostrictive delay line (1) over the whole area of the range to be measured; a magnet (2) slidably disposed along the length of said ultrasonic magnetostrictive delay line (1); a drive pulse generator (5,9) for generating a drive pulse, the drive pulse being supplied to said wave transmitter (4) so as to cause an ultrasonic wave to propagate through said ultrasonic magnetostrictive delay line (1) and be reflected from said open end as a reflected wave; and means (7) for measuring the position of said magnet (2) from said one end of said ultrasonic magnetostrictive delay line (1) on the basis of the delay time of a detection pulse relative to said drive pulse, the detection pulse being generated in said detection coil (3) by said magnet (2) in response to said drive pulse applied to said wave transmitter (4); characterised by phase matching means (8,9) for matching at said one end of said delay line the phase of said transmitted ultrasonic wave with the phase of said reflected wave by making the period of said transmitted ultrasonic wave coincident with the period of said reflected wave when said reflected wave reaches said one end of said delay line, thereby synchronizing the reflected wave with the transmitted wave at said one end of said delay line. A length measuring apparatus according to claim 1, further comprising reflected wave detecting means (8) which detects occurrence of said reflected wave at the said one end of said delay line. A length measuring apparatus according to claim 2, further comprising means (7) for calculating a ratio between a time from said drive pulse to the next drive pulse and a time from said drive pulse to said detection pulse and means (7) for outputting a position signal corresponding to a distance from said wave transmitter to said magnet.

Inventors: SAWA MASAAKI, SAWA, MASAAKI
Applicants: MACOME CORP, MACOME CORPORATION
Application year: 1994
Main classification: G01D5
IPC codes: G01B17, G01D5